1. Field of the Invention
The invention relates to control of the torque of a fastener tightened by an impact tool. More specifically, the invention is a method and apparatus which utilizes assumptions of fastener rotational inertia and joint rate to allow accurate control of the break-away torque or bolt tension of a fastener tightened by an impact tool without the need for accurate knowledge of fastener specifics.
2. Description of the Related Art
Impact tools, also known as impulse tools, are commonly used in the assembly of large fasteners, such as automotive wheel lug nuts, as they are able to deliver large amounts of torque yet are physically compact. Such tools operate by applying impacts or pulses of torque, i.e. torque high enough in amplitude to overcome the static friction of the fastener, and thus turn the fastener, yet short enough in duration such that the average torque felt by the operator is such that the tool is able to be operated manually. Because there is little correlation between the torque within the fastener applied by the tool and the torque felt by the operator, impact tools have not been used where accurate control of the fastener torque is important. Rather, controlled-torque assembly processes have been performed manually by an operator with a torque wrench, or in an automated system with a torque-monitored, (non-impact) motor-driven tool. However, these tools are not practical for assembly of large, high-torque fasteners, such as automotive wheel lug nuts.
If an impact tool is equipped with a torquemeter on the tool output shaft and the tool is used to tighten a fastener, the torquemeter will observe the torque pulses being delivered to the fastener. Each pulse will have roughly the same pulse width and torque amplitude. Taken individually, these pulses do not provide information as to the torque within the fastener. In other words, the non linear nature of the tightening process using impact tools makes it difficult to determine the instantaneous torque within a fastener. Accordingly, torque control of impact tools has had limited success.
It is an object of the invention to facilitate torque control of an impact tool.
It is another object of the invention to apply measurement of torque within the output shaft of an impact wrench to a system controlling the break-away torque within the fastener being tightened.
It is another object of the invention to control the torque of an impact tool accurately independent of the fastener being tightened.
To achieve these and other objects, a first aspect of the invention is a method for determining fastener torque comprising the steps of applying torque pulses to a fastener, measuring the amplitude and duration of each torque pulse, and processing the values of amplitude and duration of the pulses to obtain the torque on a fastener.
A second aspect of the invention is an impact tool comprising a body, an output shaft adapted to be coupled to a fastener, means for applying torque pulses to the output shaft, a torque transducer coupled to the output shaft, and means for processing the output of the torque transducer to obtain torque on the fastener.
A third aspect of the invention is a controller for an impact tool comprising a substraction circuit having an output, a first input and a second input, the first input being configured to accept a value representing calculated torque on a fastener being tightened by the impact tool and the second input being configured to accept a value of torque impulse being applied to the fastener, a velocity circuit having an output and an input coupled to the output of said substraction circuit and configured to integrate the value of the output of the substraction circuit over time to obtain a value indicating angular velocity of the fastener, a torque circuit having an output and an input coupled to the output of the velocity circuit and configured to integrate the value of the output of the velocity circuit over time to obtain the value indicating calculated torque on the fastener, the output of the torque circuit being coupled to the first input of the substraction circuit, and a threshold comparing circuit having an input coupled to the output of the torque circuit and being configured to generate a control signal for controlling the impact tool when a predetermined relationship between the value of the output of the torque circuit and a threshold value exists.
A fourth aspect of the invention is a retrofit system for an impact tool of the type comprising a body and an output shaft adapted to be coupled to a fastener. The retrofit system comprises a shaft extension having a first end and a second end, the first end being adapted to be coupled to the output shaft and the second end being adapted to be coupled to the fastener, a torque transducer coupled to the shaft extension, and means for processing the output of the torque transducer to obtain torque on the fastener.